Yellow baboonPapio cynocephalus

SOCIAL ORGANIZATION AND BEHAVIOR

Like many other cercopithecines, yellow baboons live in
multi-male/multi-female
groups in which the females remain in their
natal groups for their entire lives
and males emigrate into a new social group to breed when they reach adult size
(Samuels et al. 1987; Bentley-Condit & Smith 1999; Combes & Altmann
2001; Silk et al. 2004). Group size varies from 17 to 77, with the average
group size at Amboseli National Park being 39 individuals (Samuels & Altmann
1991). There are slightly more adult females in the group than adult males.
At Amboseli, baboon density is about 1.15 individuals per square kilometer
(.715 individuals per square mile) (Samuels & Altmann 1991). In areas where
food availability, including agricultural foods and human refuse, is high,
group size is larger and grows more quickly compared to wild-feeding groups.
Group size increases through
recruitment and immigration of adult males (Samuels & Altmann
1991).

Females remain in their natal groups with their close female relatives.
Dominance
hierarchies exist among
matrilines and
young female baboons inherit the rank of their mothers (Bentley-Condit & Smith
1999). Dominant females assert their rank over submissive females by threats,
mild aggression, biting, chasing, displacing at feeding sites, and fighting.
Submissive animals respond by averting their head and body, avoiding the dominant
animal, crouching, and screaming (Bentley-Condit & Smith 1999). Once dominance has been established between animals, though, the dominant individuals
do not have to constantly behave aggressively to maintain their status as lower-ranking
individuals respect the dominance hierarchy without being threatened (Altmann pers. comm.).
When two
females are involved in an
agonistic
dispute, another female will sometimes intervene and lend support to one of the
females. These coalitions between female yellow baboons are primarily based
on
kinship and relative dominance rank (Silk et al. 2004). For example, young
females are supported by their mothers and siblings while closely related adult
females support one another during agonistic encounters. A female will not
lend support to her relative during a dispute when both females involved are
higher-ranking (Silk et al. 2004). Higher-ranking females accrue more benefits
from maintaining their rank and mediating conflict between lower-ranking females
than vice versa. Given priority access to scarce resources and enjoying higher
reproductive success, high-ranking females protect their dominance status through
the interactions of lower-ranking females (Samuels et al. 1987; Bentley-Condit & Smith
1999). Female rank remains stable for long periods but is very occasionally punctuated by rapid
periods of change in which low-ranking females assume high-ranking positions.
Some of the conditions under which this may occur includes the death or disappearance
of a dominant female who can no longer can support her daughters or sisters
in agonistic interactions and therefore the lower-ranking female wins, the
increase in size of a subordinate matriline such that the lower-ranking females
outnumber the higher-ranking females and can dominate in physical interactions,
or when a large number of juvenile females reach adolescence and displace or
outcompete older, higher-ranking adult females (Samuels et al. 1987).

Males disperse from their natal groups around 8.5 years of age (Alberts & Altmann
1995a). It is at this age that they have reached their full size and fighting
ability even though they are capable of reproducing long before this age (Altmann
et al. 1988). Male yellow baboons also exhibit a dominance hierarchy in which
the largest, most capable fighters, usually the youngest immigrants, are the
most dominant. Subordinate males flinch or move out of the way of a dominant
male, jump back when approached, grimace, and scream (Noë & Sluijter
1995). Adult
and subadult
males are dominant over juvenile males and females and adult females,
regardless of rank (Hausfater 1975; Bentley-Condit & Smith 1999). One benefit
of high rank among males is higher levels of reproductive success, on average,
than lower-ranking males (Alberts et al. 2003).

Before joining a new group, an emigrating male spends a period of time alone,
usually lasting two months (Alberts & Altmann 1995a). This is a dangerous
time for a young male because when moving solitarily on the savanna,
yellow baboons are subject to higher levels of predation. When a male attempts
to enter a new group, he immediately begins challenging other adult males in
agonistic encounters in order to obtain reproductive opportunities with adult
females (Drews 1996; Alberts & Altmann 2001). These fights can lead to serious
injuries in males including cuts, punctures, and scratches from the opponent's
formidable canine teeth as well as broken bones. Indirect injuries include
loss of eyesight, bacterial infection, increased vulnerability to
parasites,
decreased ability to
forage,
and increased vulnerability to predators (Drews 1996).
At one site, it was observed that
when males are injured, they drop approximately one step from their
previous rank and if the newcomer does not win a fight with an established
adult male, he may be evicted from the group (Drews 1996). Like females, yellow
baboon males form coalitions and support each other during fights. These coalitions
are not based on kinship, however, as males within a group are not likely
to be related, but rather are based on fighting ability. Middle-ranked males
with average individual fighting abilities form alliances with similarly ranked
males in order to dominate in interactions with higher-ranked males (Noë & Sluijter
1995). Once they have achieved
top rank
within the group, male yellow baboons
maintain it for an average of two years before being displaced by a younger
male, but tenure can be as short as one month and as long as 11.5 years (Alberts & Altmann
1995a). Males transfer groups throughout their lives and the average residency
duration is 2.8 to 4.25 years (Altmann et al. 1988).

REPRODUCTION

Females reach reproductive maturity around 4.5 to five years of age in the
wild, and a female first gives birth around six to 6.5 years of age. In captivity,
yellow baboons reach adolescence earlier, around three to 3.5 years of age
and can give birth within a year after puberty (Altmann et al. 1988; Rhine
et al. 2000). The average
ovarian
cycle lasts about 33 days in yellow baboons
and is characterized by
anogenitaltumescence and
menstruation (Hausfater 1975). The peak of swelling is concurrent with
ovulation and
is a reliable sign to adult males and other females of the reproductive state
of the female (Hausfater 1975). One advantage of being a daughter in a high-ranking
matriline is that high-ranking daughters reach sexual maturity and reproduce
at a younger age, almost an entire year earlier, than low-ranking daughters,
increasing their overall potential
fitness.
Once they reach sexual maturity, female yellow baboons reproduce consistently
until old age and associated problems prevent them from reproducing (Rhine et
al. 2000). Yellow baboons do not exhibit birth seasonality as females mate and
give birth throughout the year. Because of the somewhat marginal habitat in which
they live, nutritional stress plays an important part on the regularity of ovarian
cycling among females. Female yellow baboons are more likely to cycle during
the rainy season, when resources are readily available, compared to the dry season
(Altmann et al. 1988).
Gestation
lasts 180 days and the average
interbirth
interval is 1.78 years (Altmann 1970;
Rhine et al. 2000). Females experience
lactational
amenorrhea and the ovarian
cycle returns as the infant begins to be weaned (Altmann
et al. 1988).

Physical development in males is slower than in females so that when young
yellow baboon females are first conceiving, around age six, their male counterparts
are only about half the adult size and are too low-ranking to mate within the
group (Altmann et al 1988). When males reach their full size, they usually
leave their natal group. Males that do not leave their natal group will stay
and reproduce for several years before emigrating, potentially breeding with
close relatives. One reason a young male risks inbreeding is the high cost
of immigrating into a new group; when males disperse they spend a period of
time alone and are subject to predation. Additionally, time spent alone translates
into lost mating opportunities (Alberts & Altmann 1995a). Males experience
their first sexual consortship around eight years of age (Alberts & Altmann
1995b).

Because baboons mate throughout the year, and females are not likely to be
fertile at the same time of year, males are able to monopolize mating opportunities
with a single female through consortship behavior and mate guarding. High-ranking
adult males persistently follow estrous females and repeatedly mate with them
during the most fertile phase of their cycle. These consortships often result in the pair lagging
behind the rest of the group during traveling and foraging, peripheralization
of the couple while the female is most receptive, and intense fights as the
dominant male prevents other males from mating with the female (Rasmussen 1986;
Alberts et al. 1996). Females exercise mate choice in the formation of these
consortships; they can be eager to form a bond with a male, approaching and
following him, sexually presenting, and allowing
copulations or they can avoid
an approaching male by walking away, rarely presenting, and not tolerating
copulations (Altmann et al. 1988). When two yellow baboons form a consortship,
it may last a morning, an afternoon, or an entire day, and females can have
multiple mates during the
estrus period (Hausfater 1975). Females do not always
choose males based on rank but on mutual cooperation or "friendship" formed
by grooming, maintaining close proximity, and infant handling (Rasmussen 1986;
Altmann et al. 1988). Because the highest-ranking males in a group are likely
to be new immigrants, females that have not established relationships with
new males avoid them, especially if they have young offspring. Lower-ranking
males form alliances and can harass newly immigrated but dominant males and
protect adult females with which they have bonds. Overall, though, dominance
rank of a male indicates reproductive success so that high-ranking males have
more mating opportunities, more offspring, and increased fitness compared to
lower-ranking males (Alberts et al. 2003).

PARENTAL CARE

Females are the primary caregivers to their dependent offspring, but males
contribute to the survival of infants in direct and indirect manners (Altmann
1980; Altmann et al. 1988; Stein 1984). At Amboseli National Park, survival
rates of the first year are around 79%, and maternal effects such as rank influence
the survival of individual infants (Altmann & Alberts 2005). Shortly after
birth, baboon infants cling to their mothers largely unaided and begin to nurse.
For about the first week of life, the mother offers a supportive embrace to
the infant to ensure that it does not lose its grip when she is moving (Altmann
1980). For the first two weeks of life, the infant will not move from the mother,
but will increasingly begin to move around on her ventrum. They have poor
motor control in the early weeks of life, and any locomotory attempts usually
end with the infant falling over after the first few steps. By the end of the
second week, the infant attempts to break contact with its mother, but the
mother is extremely attentive and at any sign of distress or any change in
the surroundings, including approaching group members, the mother immediately
retrieves her offspring (Altmann 1980). Generally speaking, higher-ranking
mothers are more permissive with their infants than lower-ranking females.
High-ranking females allow their infants to be handled by other group members
and groomed by others more frequently compared to low-ranking ones (Altmann
1980). A new infant attracts the attention of all group members, regardless
of the mother's rank, and this is one reason that permissiveness and protectiveness
are correlated with rank. Yellow baboon infants of low-ranking mothers are
occasionally
stolen or kidnapped by higher-ranking females. The mother is helpless
to retrieve her infant because she is comparatively lower in the dominance
hierarchy and is not likely to have female support to fight with higher-ranking
females (Altmann 1980).

The infant rides ventrally for
two to three months but then begins to move around to ride on the mother's dorsal
side. At around two months of age, the
mother begins to initiate breaks in contact with her infant, and by three or
four months of age, infants spend increasing amounts of time in peer interactions,
including play. Their locomotor skills have increased greatly by this point
and they are able to climb some trees. They are still highly dependent on their
mothers for food and transportation, especially if the group is traveling quickly,
but they become increasingly independent as they age (Altmann 1980). They increase
the amount of solid food consumed around four months of age, and within one
year most yellow baboon infants can survive without their mother, though they
continue to nurse occasionally and sleep in the mother's sleeping tree until
a younger sibling is born (Altmann 1980; Rhine et al. 1985). The amount of
time in contact with the mother begins at 100% in the first week of life and
decreases by eight or nine percent each consecutive month of life. The mother
increasingly resists an infant's attempts to be carried and nurse as the infant
ages such that by six to eight months of age, she often dislodges the infant
while walking and ignores the infant when it attempts to make contact (Altmann
1980). Weaning is almost completed by the end of the first year, but until
another sibling is born, a young yellow baboon will nurse during times of extreme
stress or alarm, as a form of comfort (Altmann 1980; Rhine et al. 1985).

COMMUNICATION

Visual communication among yellow baboons is important as a measure to deter
agonistic interactions involving physical fighting, especially among males.
Fighting is often very costly in terms of injuries that can lead to a host
of other secondary problems such as infections, permanent injury, inability
to forage, and inability to travel with the group, leading to higher likelihood
of predation. Baboons therefore have highly ritualized signals which communicate
threat without escalating to physical fighting (Drews 1996). These include
intense staring, eyelid displays where one male blinks slowly, exposing his
whitish eyelids to his potential competitor, ground slapping, audible chewing
or teeth grinding, yawning (which displays the formidable canine teeth), eyebrow-raising,
ear flattening, jerking of the head down and forward, piloerection,
rearing onto the hind legs, and shaking of rocks and branches (Hall & DeVore
1965; Altmann 1967; Drews 1996). Females also use the yawn display during times
of social tension (Hall & DeVore 1965). Some friendly signals seen in baboons
include lipsmacking, grinning, looking over one shoulder, presenting any body
part for grooming, and ear-flattening (Hall & DeVore 1965).

Vocalizations among yellow baboons include those given only by adult males,
those heard from any adult baboon, and juvenile calls. Some calls given only
by adult males include "barks," loud calls that can be heard for distances
over.8 km (.5 mi) and which are given as a reaction to dangerous situations
such as the presence of predators, "grunts," heard as males begins to threat
display, "roars," loud vocalizations that are heard during physical confrontations,
and "grating roars," resonant vocalizations with low intensity given by dominant
adult males (Hall & DeVore 1965). Calls that can be given by any adult
yellow baboon include "screeching," heard as a submissive animal retreats from
a more dominant group member; "shrill barks," given in response to the sudden
appearance of animals of different species, including predators; "dog barks,"
heard when one animal is separated from the rest of the group, and "grunts,"
heard when the group feeds together and as they gather in the sleeping grove
at night (Hall & DeVore 1965). One vocalization given only by adult females
in estrus is the "muffled growl," a sound made as the female inhales and exhales
with her mouth almost closed and puffing out her cheeks. This is also called
a "copulation call" as it is heard during and after copulation (Hall & DeVore
1965; Semple et al. 2002). Juveniles, including infants, also make vocalizations
unique to their age-class, including "chirplike clicking," heard when the young
baboon is separated from its mother or when it is frustrated and "ick-ooers,"
heard in similar situations (Hall & DeVore 1965).

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